We show that the critical dynamics of dense pseudospin systems interacting via retarded long-range forces (e.g., acoustic phonons) gives rise to the appearance of a $Z$ branch in the central-peak dispersion which should be observable by light-scattering techniques. The evolution of an initially nonconserved quantity towards a hydrodynamic regime leads to liquidlike behavior of the solid for a range of wave vectors.

• Received 3 May 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.478